1. Field of the Invention
The present invention relates to an optical fiber amplifier. More Particularly, the present invention relates to an optical fiber amplifier that can attain a sufficient gain shift effect, a small noise property and a high operation efficiency at the same time, and an optical amplifier having the same.
2. Description of the Related Art
Traditionally, a rare earth element dopant optical fiber amplifier has been vigorously researched and developed as a key device in a large capacity wavelength multiplexing communication system.
An optical fiber amplifier in which Tm (thulium) that is one of rare earth elements is doped has been recently researched and developed in order to effectively use an entire area of a low loss band (1450 to 1650 nm) of a quartz optical fiber transmission path as a wavelength resource. developed in order to effectively use an entire area of a low loss band (1450 to 1650 nm) of a quartz optical fiber transmission path as a wavelength resource.
One example of this Tm dopant optical fiber amplifier reports that a gain band can be generated in a 1.47 μm band by using a laser light of a 1.05 μm band as an exciting light source (refer to Japanese Patent Gazette (Jp-B-2688303).
Also, another example of the Tm dopant optical fiber amplifier reports that a gain band can be shifted to a longer wave side, from the original Tm gain band (a 1.47 μm band) to a 1.49 μm band (hereafter, referred to as a gain shift) by using the lights of two wavelengths of 1.05 μm and 1.56 μm as an excitation (refer to Optics Letters vol.24 p.1684, 1999).
FIG. 1 shows the transitions of a first excitation light having a wavelength of 1.05 μm and a second excitation light having a wavelength of 1.56 μm, in a Tm energy level.
FIG. 2 is a configuration view showing an example of a conventional two-wavelength excitation Tm dopant optical fiber amplifier. Tm dopant optical fiber modules 31a to 31c are connected in series. In order to introduce an excitation light, a first exciting light source 32a having a wavelength of 1.05 μm and a second exciting light source 33a having a wavelength of 1.56 μm are connected at the input side of the Tm dopant optical fiber module 31a through a proper wavelength multiplexing optical coupler (not shown). Similarly, a first exciting light source 32b and a second exciting light source 33b are connected at the input side of the Tm dopant optical fiber module 31b through a proper wavelength multiplexing optical coupler (not shown).
In this two-wavelength excitation Tm dopant optical fiber amplifier, an input signal 34 inputted to the Tm dopant optical fiber module 31a is amplified while sequentially passed through the Tm dopant optical fiber modules 31a to 31c, and outputted as an output signal 35 from the Tm dopant optical fiber module 31c. 
In this two-wavelength excitation Tm dopant optical fiber amplifier, a gain of 25 dB and a noise factor (noise figure) of 5 dB are attained in a band between 1475 and 1510 nm.
Typically, as an excitation configuration in the optical fiber amplifier, there are a forward excitation for introducing an excitation light in a direction equal to that of a signal light from a forward portion (an input side) of an optical fiber, a backward excitation for introducing an excitation light in a direction opposite to that of a signal light, and a two-way excitation for jointly using them.
A feature of the forward excitation lies in a small noise. A feature of the backward excitation lies in a high output and a high efficiency. And, a feature of the two-way excitation lies in the fact that it has both the features of the forward and backward excitations. Especially, in the backward excitation, a distribution of an excitation strength in a longitudinal direction in the Tm dopant optical fiber coincides with a distribution of a strength of a signal which is amplified while it is transmitted. An operation efficiency is high since the excitation strength is high at a portion where the signal strength is high.
FIG. 3 is a configuration view showing a conventional example composed of a plurality of stages of optical fiber amplifiers in which rare earth elements, such as Er (erbium) and the like, are doped. A former stage 41 is composed of a rare earth dopant optical fiber module 51a, an exciting light source 52a for introducing an excitation light and an isolator 53a. A latter stage 42 is composed of a rare earth dopant optical fiber module 51b, exciting light sources 52b, 52c for carrying out a two-way excitation and an isolator 53b. The former stage 41 and the latter stage 42 are connected in series through an isolator 53c. 
In this rare earth dopant optical fiber amplifier, an input signal 43 inputted through the isolator 53a to the rare earth dopant optical fiber module 51a is amplified while sequentially passed through the rare earth dopant optical fiber modules 51a, 51b. Then, it is outputted as an output signal 44 from the isolator 53b. Accordingly, the former stage 41 is forwardly excited by the exciting light source 52a so that the small noise is attained. The latter stage 42 is excited in the two-way by the exciting light sources 52b, 52c so that the high gain and the high efficiency are attained.
The Er dopant optical fiber amplifier is detailed in “Electronics Letters” vol.34, p.567, 1998, and “Electronics Letters” vol.34, p.1747, 1998.
By the way, in the above-mentioned conventional two-wavelength excitation Tm dopant optical fiber amplifier, an efficiency (a ratio of an amplifier output power to an excitation power) is low, such as about several percents or less. Thus, even if the excitation power is about 500 mW, an amplifier output in a signal wavelength of 1500 nm is about +6 dB at most. This results in a problem of a lack of the amplifier output.
For example, when an amplifier output necessary for a transmitter, a relay or the like is roughly calculated under an assumption of a typical wavelength multiplexing light communication system of about 16 waves, if an input power per channel of a signal is −25 dBm/ch and a fiber amplifier gain is 25 dB, an amplifier output of +12 dBm is required. Thus, the amplifier output becomes short in the current system.
Here, it is considered that the reason why the efficiency of the two-wavelength excitation Tm dopant optical fiber amplifier is lower than that of the Er dopant optical fiber amplifier lies in the employment of the forward excitation configuration. Thus, in order to improve the efficiency, it is considered to apply the backward excitation for introducing the excitation light from behind (output sides of) the Tm dopant optical fiber modules 31a to 31c or the two-way excitation, even in the two-wavelength excitation Tm dopant optical fiber amplifier.
However, in the conventional two-wavelength excitation Tm dopant fiber amplifier, the execution of the backward excitation or the two-way excitation with the two-wavelength causes the noise factor to be extremely deteriorated (increased), which results in the extremely poor property as the optical fiber amplifier.